1. Field of the Invention
The present invention relates to an electro-optical device which provides fine display images. More specifically, it relates to a dispersion type liquid crystal electro-optical device (polymer-dispersion liquid crystal (PDLC) electro-optical device) comprising a plurality of electrodes having incorporated therebetween light shielding portions having specified height.
2. Prior Art
Liquid crystal displays are now replacing conventional display devices such as televisions equipped with cathode ray tubes (CRTs), because liquid crystal displays are thinner, compact, light weight, and consume low power. With progress in technology, liquid crystal displays operating in an STN (super twisted nematic) type mode have become dominant over those of a TN (twisted nematic) type mode, and also their use has extended widely from watches to the field of word processors and liquid crystal television sets.
It is indispensable in those displays, however, to incorporate one or two polarizer sheets in the liquid crystal cell. Because the light transmittance per polarizer sheet is as low as in the range of from 40 to 50%, the light transmittance of a TN type display using the polarizer sheet(s) as a result falls to a low 10 to 30% even in the bright state (the state of transmitting light). Accordingly, an additional backlighting is requisite to compensate for the optical loss to lighten the dark display.
Recently, polymer-dispersed liquid crystals free from the aforementioned polarizer sheets and which provide bright displays of high contrast images are studied more extensively. This type of liquid crystal comprises an electro-optical modulating layer based on a transparent solid phase polymer having dispersed therein granular or sponge-like nematic, cholesteric, or smectic liquid crystal materials. A known process for fabricating an electro-optical device of this type comprises dispersing liquid crystals in a polymer by encapsulating the liquid crystal, and then establishing the polymer as a thin film on a film or a substrate. Proposed materials for encapsulating the liquid crystal include gelatin, gum arabic, and poly(vinyl alcohol).
For instance, liquid crystal molecules being encapsulated in poly(vinyl alcohol) and having a positive dielectric anisotropy arrange themselves along a single direction on applying an external electric field, in such a manner that the molecules orient their major axes along the direction parallel to the direction of the electric field. If the refractive index of the oriented liquid crystal molecules is equal to that of the polymer, a light can pass straight forward through this portion. Hence, in this case, light can be transmitted through this portion.
If no external electric field is applied to the liquid crystal, the liquid crystal molecules do not take a particular arrangement and the major axes thereof yield a random orientation. Accordingly, the refractive index of the polymer becomes different from that of the liquid crystal. Thus, in this case, a light passing through this portion is scattered that the liquid crystal exhibits an opaque (milky white) appearance.
PDLC electro-optical devices are not only confined to those comprising encapsulated liquid crystals as described above, but also various others of this type are known to the present. PDLC electro-optical devices include those comprising an electro-optical modulating layer which takes advantage of, for example, an epoxy resin having dispersed therein liquid crystals, a phase separation between the liquid crystal and a photo-curable resin, a three-dimensionally connected polymer having impregnated with liquid crystal, and a cholesteric/nematic phase transition. All those are collectively referred to as PDLC electro-optical device in the present invention.
In a PDLC electro-optical device of this type, as mentioned hereinbefore, the liquid crystal molecules arrange themselves in such a manner that the major axes of the molecules take a random orientation. Accordingly, a light incident to this portion is scattered and changes its optical path upon passing through the liquid crystal system. A PDLC electro-optical device having a plurality of electrodes and driven by a matrix provides superfine displays. Those displays may be driven in various manners, for example, by a simple matrix obtained by patterning a transparent electrode and by thin film transistors (TFTs) provided in each of the pixels. At any rate, because such PDLC electro-optical devices deal a large information with a superfine display, they comprise pixels in high integration. Hence, the distance between the pixels becomes very short.
In a well-known type of liquid crystal displays driven by TFTs, black stripes as light shields are provided between the pixels to prevent light from leaking, because the liquid crystal cannot be controlled at portions between the pixels. The black stripes are provided, for example, by printing black pigments, by patterning pigments using photographic processes, and by film depositing and patterning light-shielding metals such as aluminum and chromium. Those black stripes prevent the leak light from being transmitted between the pixels.
In a conventional type of liquid crystal electro-optical device using a nematic liquid crystal, the light incident to the device from the light source provided on the back of the device passes straight forward through the pixels without considerably deviating its light path. An `OFF` state can be selected from an `ON` state by cutting off the light path with a plane of a desired pixel. In this manner, the light corresponding to the neighboring pixels can be distinguished by the pixel having turned `OFF` and a black plane (thin) stripe which prevents light leakage from occurring between the pixels. A thin film of a metal having a thickness of from 500 to 1000 .ANG. and having a low light transmittance works sufficiently as a light shield.
A PDLC electro-optical device, however, is different from the aforementioned liquid crystal electro-optical device operating on a TN mode. That is, the 'OFF's and 'ON's of a PDLC electro-optical device are realized by whether the incident light is scattered or is allowed to pass straight forward through the cell. In a PDLC electro-optical device having a 500 .ANG. thick thin film of a metal as a black stripe, for example, the light being scattered from the neighboring pixels mix to result in not only leak light but also blurring of image. Accordingly, the black stripe no longer functions as a light shield.
Optical switching of a PDLC electro-optical device is realized between a state of scattering the light and a state of transmitting the light. The former state can be obtained by changing the light path of the light passing through the electro-optical modulating layer by taking advantage of a random orientation of liquid crystal molecules having anisotropy of refractive index. The latter state can be achieved by applying an external electric field to the electro-optical modulating layer so as to arrange the liquid crystal molecules in one direction, so that the light passing through the electro-optical modulating layer may proceed straight forward along the direction of incidence without being scattered. Accordingly, the black stripes for use as light shields are not effective for a PDLC electro-optical devices. That is, the problem resides in the fact that a black stripe, which is a plane extending in two dimensions, is used as a light shield for a light spreading in three dimensions.